The present invention relates to a high-voltage voltage transformer, especially to a combined high-voltage current- and voltage-transformer of head-type construction with a core at high voltage that is surrounded coaxially by a high-voltage winding having a slotted metal shield and with a low-voltage winding also coaxially surrounding the high-voltage winding as well as with a coaxial shielding electrode at ground potential and surrounding the high- and low-voltage winding.
Such high-voltage voltage transformers are known as such. With combined current- and voltage transformers of head-type construction, it is customary to mount the active parts of the transformer consisting of a core and of a high- and low-voltage winding on an insulating column and to surround the same with a head housing. The insulating column is secured at a base part which carries the terminal box with the terminal panel for the lead-out connecting lines of the transformer. The secondary lead-out connecting lines from the head housing of the transformer to the terminal panel are therefore relatively long. This is true in particular for combined high-voltage current- and voltage-transformers of the type of construction according to the older, non-prepublished patent application P 36 08 390.9, which corresponds to U.S. Pat. No. 4,731,599, of the assignee of this application in which the voltage transformer is arranged above the current transformer.
As a result of the large distance conditioned by reason of the necessary fastening elements for the low-voltage winding between the metal shield of the high-voltage winding and of the low voltage winding, the capacity between the metallic shields of the two windings is relatively small.
It has now been discovered with such arrangements that during transient, especially high frequency occurrences, in particular triggered by switching operations, the metal shield of the high voltage winding of the voltage transformer which as such is at low voltage potential, can be raised to a high potential of some 10 kV up to 100 kV or more.
The consequence is that during the occurence of high potentials by reason of high frequency transient surge or over-voltages at the metal shield of the high-voltage winding, these high frequency voltages may be coupled into the low voltage winding. This may even lead to the destruction of such high-voltage voltage transformers if the insulation between the metallic shielding and the low-voltage winding is destroyed.
Accordingly, it is the object of the present invention to so improve a high-voltage voltage transformer, especially a combined high-voltage current- and voltage-transformer of head-type construction of the aforementioned type, that coupling effects of high potentials, especially triggered by high frequency transient switching operations, onto the secondary side of the transformer, especially onto the secondary connecting terminals can no longer occur to any troubling extent.
The underlying problems are solved according to the present invention in that the metal shield is connected with the connecting terminal for the high-voltage winding, a discharge electrode concentrically surrounding the metal shield is provided insulated with respect to the metal shield at a small distance of up to 5 mm. from the metal shield, the discharge electrode is electrically conductively connected low in inductance with shield electrode by way of one or several connecting lines.
The present invention assures that a very large capacity is formed between the metal shield of the high voltage winding and the discharge electrode of the voltage transformer. The discharge electrode is electrically connected with the outer shielding electrode along the shortest path and therewith extraordinarily low in inductance. A rise of the potential of the metal shield of the high voltage winding to non-permissively high values is thereby reliably prevented and a coupling of non-permissively high transient over-voltages into the low voltage winding is prevented.